I believe this is the reason: Imagine, you work in bigCorp. You have company email address: my-name@bigCorp.com bigCorp pays for your access to SaaS service.

I believe this is the reason: Imagine, you work in bigCorp. You have company email address: my-name@bigCorp.com bigCorp pays for your access to SaaS service. You switch jobs, your email is revoked/removed. You can not log in anymore. If there was no 2FA via email - you still can access service with email+password in case they failed to remove your access to specific service. If all services use 2FA via email - bigCorp has less access problems. That is also partly related with SAML/SSO lack of "sign off".